Networks, in particular mobile networks, are in search of appreciable gains in terms of capacity, reliability, consumption and the like. The transmission channel of a mobile network is presumed to be difficult and leads to relatively mediocre transmission reliability. Significant progress has been made in recent years, in regard to coding and modulation, especially in respect of considerations of consumption and capacity. Indeed, in a mobile network where several senders/receivers share the same resources (time, frequency and space) it is necessary to reduce to the maximum the transmit power.
This reduction runs counter to the coverage and therefore to the capacity of the system and more generally to its performance.
To increase the coverage, enhance the reliability of the communications and more generally to improve performance, one approach consists in employing relays to increase the spectral efficiency and therefore improve the transmission efficiency and the reliability of the systems. The topology of MARC systems, illustrated by FIG. 1, is such that the sources, nodes S1 and S2, broadcast their coded information sequences for the attention of the relay R and of the recipient D. The relay decodes the signals received from the sources S1 and S2 and re-encodes them jointly while adding inherent redundancy creating a spatially distributed network code. At the destination D, the decoding of the three spatially distributed coded sequences, comprising the two coded sequences received directly from the sources S1 and S2 and the coded sequence from the relay, relies on channel/network joint decoding algorithms.
Network coding is a form of cooperation according to which the nodes of the network share not only their inherent resources (power, band, etc.) but also their calculation capacity, so as to create increasingly powerful distributed coding as the information propagates through the nodes. It brings substantial gains in terms of diversity and coding and therefore of reliability of transmission.
Two types of operation for the relay are distinguished: half-duplex mode and full-duplex mode.
According to the half-duplex mode, two transmission phases are distinguished which correspond to different transmission intervals since the relay is incapable of receiving and sending simultaneously. During the first phase which comprises the first transmission intervals (time slots), the two sources send but not the relay. The relay decodes/re-encodes jointly so as to deduce the signal to be sent during the next transmission intervals. During the second phase which comprises the second transmission intervals, the relay sends the signal determined during the first transmission intervals and the sources send the second parity sequences relating to the same information as sent during the first transmission intervals. Relays of half-duplex type are attractive on account of a simple communication scheme and on account of the ease of implementing them and of their reduced cost which stem therefrom.
According to the full-duplex mode, the relay receives the new blocks of information from the two sources and simultaneously transmits to the recipient its code word based on the blocks previously received. In comparison to the half-duplex relay, the full-duplex relay makes it possible to achieve a greater capacity.
The articles C. Hausl, F. Schrenckenbach, I. Oikonomidis, G. Bauch, “Iterative network and channel coding on a Tanner graph,” Proc. Annual Allerton Conference on Communication, Control and Computing, Monticello, Ill., 2005 and C. Hausl, P. Dupraz, “Joint Network-Channel Coding for the Multiple-Access Relay Channel,” Proc. IEEE SECON'06, Reston, Va., September 2006 describe a channel/network joint coding for a MARC system, illustrated by FIG. 2. The MARC system considered is such that all the links CH14, CH24, CH13, CH43 and CH23 are orthogonal, furthermore the links between the two sources and the relay are assumed to be perfectly reliable. In the patent application the link is the communication channel between two or more nodes, it may be physical or logical. When the link is physical then it is generally called a channel. The two sources S1, S2 broadcast the coded information to the relay R and to the recipient D during the first transmission phase. The relay R linearly combines the assumed perfectly decoded streams of the two users according to a linear network coding scheme. During the second phase, the relay sends an additional parity sequence to the recipient D. This channel/network joint code may be considered, once all the streams have been received, stored and reorganized at the level of the recipient, as a spatially distributed channel/network joint code which may be iteratively decoded. This joint code brings substantial gains in terms of diversity and coding.
S. Yang and R. Koetter have evaluated the performance of network coding for a MARC system, illustrated by FIG. 3, with orthogonal links but in the presence of noisy source-relay links. S. Yang, R. Koetter, “Network coding over a noisy relay: A belief propagation approach”, in Proc. IEEE ISIT'07, Nice, France, June 2007. The authors propose the “soft decode and forward” technique which relies on the generation of a discrete distribution of probabilities over the bits to be transmitted, obtained by an algorithm calculating the a posteriori probabilities (APP) over the coded bits/symbols. Each source S1, S2 generates a code word transmitted to the relay R. The relay R decodes them in the form of a logarithmic probability ratio (LLR) by using a BCJR decoding algorithm, the initials stemming from the name of its authors L. Bahl, J. Cocke, F. Jelinek, and J. Raviv, and then performs a memory-less weighted network coding corresponding to the bitwise sum modulo two (XOR operation) of the two code words received, the weighted coding consisting in generating on the basis of the LLRs L1, L2 of the two sources a third LLR LR corresponding to the XOR operation. L. Bahl, J. Cocke, F. Jelinek, and J. Raviv, “Optimal Decoding of Linear Codes for minimizing symbol error rate”, IEEE Transactions on Information Theory, vol. IT-20(2), pp. 284-287, March 1974. This third LLR is ultimately transmitted in an analog manner to the destination D. Thus, the recipient has three observations: those emanating from the two sources and the LLR. The recipient performs jointly and in an iterative manner a decoding of the streams of the source S1 and of the source S2 by utilizing the additional information provided by the relay. The article describes that, even for severely noisy links S1→R and S2→R, the network coding affords a coding gain with respect to a scheme without cooperation, therefore without any relay. The method is described in the case of a BPSK modulation and cannot be transposed to a modulation with an order greater than four since the expression calculated during the 3rd step can apply only for a modulation of order two or four (e.g. QPSK).
In these various known systems, the decoding errors are reduced solely in the absence of interference since the MARC system considered is assumed interference-free on account of the orthogonal links. Furthermore, the constraint which consists in imposing orthogonal links leads to non-optimal use of the spectral resource and therefore to a limitation of the capacity of the network.